starts then cuts out?

My 1999 G/Cherokee starts when I turn the key but as soon as I 'let go' the engine dies,if I keep it 'on' the starter then it still dies. I have checked the fuel filter & it's OK.The fuel pump is running fine, there is fuel in the system. Spark plugs have been checked and cleaned, vacuum assembly is clean and working, distributer cap & rotor arm appear to be serviceable, butterfly valve works when throttle is applied (was blackened with soot/black residue but now cleaned) If I spray 'cold start' into the intake it starts OK but then stops. I'm now at a loss as to what to do...any help would be much appreciated.